Coriolis Vibratory Gyroscopes (CVGs) are useful for measuring or detecting angular movement or acceleration and are based on vibrating mechanical elements which sense rotation. A resonator of a CVG is excited into vibration at its resonant frequency. The direction or orientation of vibration is somewhat dependent on rotation of the sensor so that the vibration can be analyzed to sense directional changes. An amplitude control loop is used to apply excitation energy to the resonator of the CVG and to sense the resulting vibration. A number of circuits are employed to excite the resonator to resonance and to produce an output indicative of rotational rate.
In certain CVGs, an amplitude control loop includes a low impedance buffer that extracts the charge from one or more capacitive pickoffs of a vibratory gyroscope to measure the amplitude of the resonator vibration. The resonator vibration responds to the amplitude of a drive signal supplied to one or more forcer electrodes to induce vibration in the gyroscope resonator. Capacitive pickoffs may be used to measure the vibratory displacement of the resonator through the electrical charge they produce. The charge from the capacitive pickoff(s) is transferred to an integration capacitor (CF) where the charge is converted to an output voltage. That output voltage is sampled and, for the anti-nodal channels, is used to measure the amplitude of the resonator vibration. A controller adjusts the drive amplitude to maintain the measured vibration amplitude at a predetermined value. Also, by measuring both in-phase and quadrature components of the anti-nodal signal, it is possible to determine frequency error (i.e. deviation between the excitation frequency and the resonant frequency). A controller can then use this information to correct the frequency and ensure the drive tracks the resonant frequency of the CVG.
The output voltage from the integration capacitor (CF) is demodulated and provided to a summer which summed in an amplitude reference signal to produce an error signal. However, the gyro scale factor (which depends on the sensor vibration amplitude) is highly sensitive to the integration capacitor in the low-Z buffer circuit. If the capacitor CF varies, then for a given vibratory displacement, the voltage generated by the buffer will vary in inverse proportion to CF. The controller only has access to the voltage, thus by maintaining constant voltage amplitude, the true vibratory amplitude will in fact vary proportionally with the value of CF. Since the scale factor of the gyro depends directly on the vibratory amplitude, the scale factor accuracy is impacted. Also, the value of CF is small and therefore susceptible to many parasitic effects.